In general, the processing of black and white silver halide photographic materials is performed in the order of development, stopping, fixing and washing.
Development is ordinarily carried out with aqueous alkaline developer compositions containing a developing agent, usually of the dihydroxybenzene type such as hydroquinone. The activity of these developing agents is greatly influenced by the pH of the solution and the optimum pH range should not significantly change during the useful life of the developer composition; therefore the composition has always to possess a high buffering capacity. The developer compositions are usually provided with antifogging agents (in particular inorganic antifogging agents such as soluble bromides and organic antifogging agents) to retard the development of non-exposed silver halide grains and decrease fog, i.e. silver formed as a result of the development of said grains.
Antioxidant compounds are usually added to the alkaline developer composition to limit oxidation of the developing agents by air. Alkaline and ammonium sulfites are the most common compounds used for this purpose, but other antioxidant compounds such as hydroxylamine and ascorbic acid can be used instead of or in combination with such sulfites.
It is a well-known phenomenon that traces of certain metal ions, such as copper and iron ions, have a catalytic effect on the aerial oxidation of the developing agents (the metal ions may have been introduced as impurities in the water or in the chemicals used to compound the developer composition). The catalytic effect of said ions on the aerial oxidation of developing agents can be reduced by suitable sequestering agents. Sequestering agents work by forming fairly stable and soluble complexes with the metallic ions so that free metal ions are present only in small amounts.
However, the activity of antioxidant compounds and sequestering agents to improve the resistance to aerial oxidation is much reduced when silver halide photographic materials are treated in continuous transport processing machines, especially at high temperatures. In this case, considerably high quantities of air are introduced into the developer composition used in processing machines, said aeration causing a rapid degradation of the developer itself.
The problem of resistance to aerial oxidation is particularly serious in developing compositions intended for processes of high contrast silver image formation in the presence of hydrazine compounds (as described for instance in U.S. Pat. Nos. 4,168,977; 4,224,401; 4,269,929; 4,272,614 and 4,323,643 and in EP Patent Application Ser. No. 155,690), wherein the relative high pH levels in developing compositions reduce their effective life.